Low pressure drop purifier for nitrogen, methane, and argon removal from syngas

ABSTRACT

An apparatus for purifying a raw syngas stream containing excess nitrogen and an ammonia process plant for manufacturing ammonia from syngas with excess air for reforming and nitrogen removal with low pressure losses is disclosed. Auto-refrigeration for cooling the syngas for cryogenic hydrogen enrichment is provided by expansion of a hydrogen-lean waste fluid stream from a distillation column

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a divisional to co-pending U.S. patentapplication Ser. No. 10/604,404, filed on Jul. 17, 2003.

FIELD

The present embodiments relate generally to methods and apparatus toimprove production of synthesis gas for manufacturing ammonia. Thepresent embodiments reduces pressure losses in a nitrogen-wash purifierunit

BACKGROUND

Processes for manufacturing ammonia from a hydrocarbon and air, via ahydrogen/nitrogen synthesis gas (syngas), are well known. Extraneoussyngas components typically include inert gases from the air and/or thehydrocarbon feed, such as argon and methane. When excess air is used inthe syngas production, nitrogen is also present in stoichiometricexcess, and must be removed from a raw makeup syngas stream or purgedfrom an ammonia synthesis loop to maintain a desired ammonia synthesisreactor feed composition.

In the prior art, some syngas production methods use excess air andcryogenic syngas purification, which relies on a syngas pressure dropupstream of purification for refrigeration. The pressure drop issubsequently made up in a compressor that raises the syngas to ammoniasynthesis loop pressure. This type of method also reduces the rate ofrecycle or purge gas flow from the ammonia reactor loop due to theupstream removal from the makeup syngas of inerts such as argon andmethane in the syngas purification.

Other methods of ammonia synthesis use high-activity catalyst in theammonia synthesis reactor. Purge gases are eliminated via a hydrogenenrichment process operating on a sidestream of the syngas recycled tothe synthesis loop compressor. The total recycle flow is roughly threetimes the volumetric flowrate of the makeup syngas.

Other methods use air separation to provide oxygen-enriched air suchthat reforming produces a synthesis gas with higher hydrocarbon slipthan in other ammonia manufacturing systems. A higher concentration ofnonreactive gas in the ammonia synthesis is managed by purging from aresidual syngas stream following recovery of ammonia product. This typeof method unloads front-end gas reforming reactors, at the expense ofincluding air separation, but ostensibly enables a smaller purge streamprocess after ammonia synthesis.

Other methods are centered on an integrated process system forsynthesizing methanol and ammonia that uses a nitrogen wash by cryogenicfractionation to purify ammonia syngas, with refrigeration suppliedexternally and providing no recovery of expansion power in the process.

The present embodiments meet these needs.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description will be better understood in conjunction withthe accompanying drawings as follows:

FIG. 1 is a schematic process flow sheet showing prior art syngaspurification using an upstream syngas feed to drive an expander andextract syngas energy as work to achieve auto-refrigeration.

FIG. 2 is a schematic process flow sheet of an embodiment of the presentinvention, using expansion of a nitrogen-rich liquid waste stream togenerate auto-refrigeration in the process.

FIG. 3 is a schematic process flow sheet showing an alternate embodimentof the present invention wherein syngas feed or liquefied waste gas canbe expanded across a liquid expander for refrigeration.

FIG. 4 is a block flow diagram of an embodiment of the invention showinglow pressure drop nitrogen removal integrated in an ammonia synthesisprocess with secondary reforming with excess air and heat-exchangingreforming.

FIG. 5 is block flow diagram of an alternative embodiment of theinvention showing low pressure drop nitrogen removal integrated in anammonia synthesis process with conventional primary steam reforming andsecondary reforming with excess air

The present embodiments are detailed below with reference to the listedFigures.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Before explaining the present embodiments in detail, it is to beunderstood that the embodiments are not limited to the particularembodiments and that they can be practiced or carried out in variousways.

The present embodiments provide methods to purify syngas. Syngas, as anexample, occurs in ammonia manufacturing processes. The methods usecryogenic distillation to purify syngas, and obtain refrigeration forthe distillation from waste fluid expansion using a liquid expander torecover mechanical work from the waste fluid. These methods reduce thepressure losses in the syngas stream and concomitantly reducecompression costs and power relative to similar prior art ammoniaprocesses utilizing nitrogen and inerts removal.

The methods are applicable in grassroots plant design, and can also beapplied to retrofit existing synthesis gas systems to improve processperformance and economics. In the retrofit, for example, the lowerpressure drop of the present embodiments can allow process modificationfor reforming with excess air and nitrogen removal from the makeupsyngas without expensive modification or replacement of the synthesisloop and/or makeup gas compressors.

In an embodiment, the present embodiments provide methods to purifysyngas, including: (a) introducing a raw syngas stream containing excessnitrogen to a feed zone in a distillation column; (b) expanding a liquidbottoms stream from the distillation column through a liquid expanderwith a work output to form a cooled waste fluid stream; (c) rectifyingvapor from the feed zone in the distillation column to form an overheadvapor stream of reduced nitrogen and inerts content; (d) cooling theoverhead vapor stream in indirect heat exchange with the cooled wastefluid stream to form a partially condensed overhead stream and arelatively warm waste fluid stream; (e) separating the partiallycondensed overhead stream into a condensate stream and a purified syngasvapor stream of reduced nitrogen and inerts content; and (f) refluxingthe distillation column with the condensate stream. The method can alsoinclude cooling the raw syngas stream by expansion across aJoule-Thompson (J-T) valve in advance of the introduction to the feedzone. Additionally, the method can include cooling the raw syngas streamin cross-exchange against the warm waste fluid stream and against thepurified syngas vapor stream. In this embodiment, adjusting the flow tothe liquid bottoms stream expansion controls liquid level in thedistillation column.

The methods can further include producing the raw synthesis gas byreforming a hydrocarbon, wherein the reforming includes autothermal orsecondary reforming with excess air. The purified syngas vapor streamcan be supplied to an ammonia synthesis loop for manufacturing ammonia.

In an embodiment, the present embodiments provide an ammonia process.The process embodiments include reforming a hydrocarbon to form syngas.The reforming can include autothermal or secondary reforming with excessair to form a raw syngas stream containing excess nitrogen for ammoniasynthesis. The process embodiments include cooling the raw syngas streamin a cross-exchanger and expanding the cooled raw syngas stream. Theexpanded raw syngas stream is introduced to a feed zone in adistillation column. The liquid bottoms stream from the distillationcolumn is expanded through a liquid expander to form a cooled wastefluid stream. The process embodiments include rectifying vapor from thefeed zone in the distillation column to form an overhead vapor stream ofreduced nitrogen and inerts content. The overhead vapor stream is cooledin indirect heat exchange with the cooled waste fluid stream to form apartially condensed overhead stream and a partially warmed waste fluidstream. The partially condensed overhead stream is separated into acondensate stream and a purified syngas vapor stream of reduced nitrogenand inerts content. The distillation column is refluxed with thecondensate stream and the purified syngas vapor stream and the partiallywarmed waste fluid stream are heated in one or more cross-exchangers.The purified syngas vapor stream is supplied from the cross-exchanger toan ammonia synthesis loop.

In an embodiment, the present embodiments can be applied to improve anammonia process that includes the steps of reforming a hydrocarbon withexcess air to form a raw syngas stream, removing nitrogen and inertsfrom the raw syngas stream by distillation. The cooling can be providedby process fluid expansion through an expander-generator, wherein anoverhead stream is partially condensed against a waste stream cooled byexpanding bottoms liquid from a distillation column, and supplyingsyngas with reduced nitrogen and inerts content from the distillation toan ammonia synthesis loop. The process embodiments can includeoptionally expanding the raw syngas stream across a Joule-Thompson valveupstream of the distillation column; and/or expanding the bottoms liquidthrough a liquid expander with a work output.

In an embodiment, the present embodiments provide for a purificationapparatus for purifying a raw syngas stream containing excess nitrogen.The embodied apparatus, can include means for introducing the raw syngasstream to a feed zone in a distillation column; means for expanding aliquid bottoms stream from the distillation column to form a cooledwaste fluid stream; means for rectifying vapor from the feed zone in thedistillation column to form an overhead vapor stream of reduced nitrogenand inerts content; means for cooling the overhead vapor stream inindirect heat exchange with the cooled waste fluid stream to form apartially condensed overhead stream and a relatively warm waste fluidstream; means for separating the partially condensed overhead streaminto a condensate stream and a purified syngas vapor stream of reducednitrogen and inerts content; and means for refluxing the distillationcolumn with the condensate stream.

In an embodiment, the present embodiments provide for an ammonia processplant. The embodied an ammonia process plants can include means forreforming a hydrocarbon to form syngas, wherein the reforming meansincludes an autothermal or secondary reformer and means for supplyingexcess air to the autothermal or secondary reformer, to form a rawsyngas stream containing excess nitrogen for ammonia synthesis;cross-exchanger means for cooling the raw syngas stream; means forexpanding the cooled raw syngas stream from the cross-exchanger; meansfor introducing the expanded raw syngas stream to a feed zone in adistillation column; means for expanding a liquid bottoms stream fromthe distillation column through a liquid expander to form a cooled wastefluid stream; means for rectifying vapor from the feed zone in thedistillation column to form an overhead vapor stream of reduced nitrogenand inerts content; means for cooling the overhead vapor stream inindirect heat exchange with the cooled waste fluid stream to form apartially condensed overhead stream and a partially warmed waste fluidstream; means for separating the partially condensed overhead streaminto a condensate stream and a purified syngas vapor stream of reducednitrogen and inerts content; means for refluxing the distillation columnwith the condensate stream; means for heating the purified syngas vaporstream in the cross-exchanger; means for heating the partially warmedwaste fluid stream in the cross exchanger; and means for supplying thepurified syngas vapor stream from the cross-exchanger to an ammoniasynthesis loop

With reference to the figures, FIG. 1 depicts an example of prior artsyngas purification PA. A syngas feed stream 10 drives expander 12,extracting syngas energy as work 14 to achieve auto-refrigeration. Thefeed stream 10 is chilled in cross-exchangers 16, 18 by indirect heattransfer with cold product streams from a distillation column 20.Between the cross-exchangers 16, 18, the raw syngas 10 is expanded in aturboexpander 12, cooling the raw syngas 10 and recovering work 14. Theexpander 12 can be bypassed or supplemented by using a Joule-Thompson(J-T) valve 22, for example during startup. The partially liquefied rawsyngas 13 from the cross-exchanger 18 enters the distillation column 20to be further cooled, partly condensed, and rectified, yielding apurified syngas stream 24 of lowered nitrogen and inerts content and ahydrogen-lean waste gas stream 26. The purified syngas stream 24 andwaste gas stream 26 pass through the cross-exchangers 16, 18 to chillthe raw syngas feed stream 10 as mentioned previously.

The waste gas stream 26 is discharged from the distillation column 20 asbottoms stream 28, flashed across level control valve 30, and used as acoolant in a heat exchanger 32 integral with the distillation column 20.The heat exchanger 32 cools and partially condenses overhead vapor fromthe column 20 to obtain syngas liquid to reflux the column 20. Themakeup syngas stream 24 is compressed for conversion in ammoniasynthesis reactors (not shown) that operate at higher pressures. Thus, apressure drop incurred by the raw syngas 10 in the purification PA mustbe recouped downstream by consuming additional power for compression.

FIG. 2 depicts an embodiment of syngas purification 34 using mechanicalexpansion of the liquid bottoms stream 28 to generate a major fractionof the auto-refrigeration in the purification process 34. A singlecross-exchanger 36 is used in place of the cross-exchangers 16, 18 ofFIG. 1, although cross-exchanger 36 can include a plurality of physicalstages. The raw syngas stream 10 is passed through valve station 38upstream of the distillation column 20. The valve station 38 can includea primary, line-size valve for flow during normal operation, and a J-Tsecondary valve for trim and/or startup for auto-refrigeration. The rawsyngas stream 10 then enters an inlet zone 40 of the column 20,preferably as a mixture of syngas vapor and liquid. In the inlet zone40, syngas liquid separates and is collected in liquid holdup zone 42.The liquid exits the column 20 as bottoms stream 28 via a lower outlet44. The column bottoms stream 28 is expanded through a liquid expander46 to auto-refrigerate the bottoms 28 and recover work 48, which can beused to drive a pump, compressor, electrical generator, or the like. Asused herein, a “liquid expander” is a work-output device that receives aliquid supply and produces a liquid or vapor effluent, preferably amixed vapor-liquid effluent. Where the effluent fluid is liquid, theliquid expander 46 can be a hydraulic turbine.

A bypass J-T valve 50 is included for gas or two-phase flow, e.g. atstartup. In operation, expansion of the bottoms stream 28 is preferablya primary source of auto-refrigeration in the syngas purificationprocess 34 of the present embodiments, whereas the expansion across thebypass J-T valve at valve station 38 is a relatively minor source. Thebypass J-T valve can be a significant refrigeration source duringstartup.

From liquid expander 46, the chilled waste fluid stream 28 enters acoolant inlet 52 of an indirect heat exchange zone 32 integral to thecolumn 20. The flow rate to the liquid expander 46 controls the liquidlevel in the holdup zone 42 and also, in part, regulates conditions inthe column 20, based on feedback from a syngas analyzer 56. Conditionsin the column 20 determine the composition of the purified syngas stream24. For example, more refrigeration reduces the nitrogen content; lessrefrigeration increases the nitrogen content. The chilled waste fluidstream 28 passes through the heat exchange zone 32, discharging from thecolumn 20 via coolant outlet 56. During transit through the heatexchange zone 32, the bottoms stream 28 cools and partially condensesoverhead vapor from the column 20.

From the inlet zone 40, syngas vapor flows upward through a contact zone58 in contact with liquid flowing downward through the contact zone 58to absorb nitrogen and enrich the hydrogen content of the vapor. At theupper end of the contact zone 58, the vapor enters a vapor riser 60 andflows to a vapor inlet zone 62 at an upper end of the heat exchange zone32. The vapor passes tube-side through the heat exchange zone 32 forpartial condensation against the waste fluid stream 28, furtherenriching the vapor in lower-boiling components. Vapor and condensateexit the heat exchange zone 32 and are separated in a knockout zone 64.Vapor exits the column 20 as the purified syngas stream 24, dischargingvia syngas outlet 66. The condensate collects in a liquid seal well 68below the knockout zone 64 and in communication with the contact zone58. The condensate overflows from the seal well 68 to flow downwardthrough the contact zone 58 to the liquid holdup zone 42 as describedpreviously.

FIG. 3 depicts another embodiment of a syngas purification process 70,in which the process PA of FIG. 1 can be modified or retrofittedaccording to the present embodiments. A bottoms liquid expander 46 isadded to auto-refrigerate the bottoms stream 28 by recovering work, forexample as power 48. A bypass J-T valve 50 is also installed, as in FIG.2. The resulting retrofit purification process 70 is comparable to theinventive embodiment of FIG. 2, but can also be operated in the originalconfiguration, if desired. For low pressure drop operation, the originalsyngas turboexpander 12 is bypassed and the valve 22 is set full open,or optionally bypassed (not shown).

In an embodiment, expansion of a liquid byproduct stream of purged gases(such as, the column bottoms stream 28) can generate a major portion ofthe auto-refrigeration required for the purification process. Thisgeneration avoids a major part of the syngas pressure loss incurred inthe prior art configuration of FIG. 1. In the prior art process PA, apressure drop of about 3.1 bars typically occurs from introduction ofthe syngas feed stream 10 to exit of the purified syngas stream 24. Mostof this pressure drop occurs across the expander 12, which drops the rawsyngas pressure by about 1.8 to 2.0 bar. In an embodiment as exampled inFIG. 2, a pressure drop from introduction of the syngas feed stream 10to exit of the purified syngas stream 24, can be limited to a range ofabout 0.75 to 1.3 bar by obtaining a major portion of the requiredauto-refrigeration effect from expansion of the column bottoms stream 28instead of from the raw syngas feed stream 10.

Referring to FIG. 4, an embodiment of an ammonia manufacturing processcan include catalytic reforming of a feed including hydrocarbon 100 andsteam 102 in a reactor/exchanger 104 of the type known under the tradedesignation KRES. Additional reforming of a feed including hydrocarbon100 and steam 102 with excess air 106 as oxidant can be effected insecondary reformer 108. The process can include high and/or lowtemperature shift conversion and carbon dioxide removal 110, methanationand drying 112, syngas purification 114 as described in reference toFIG. 2 or 3, compression 116, and ammonia synthesis 118. A purge stream120 is recycled from the ammonia synthesis 118 to upstream of the syngaspurification 114 (such as, to the methanation and drying 112). Therecycled stream 120 can be relatively smaller in mass flow rate than theraw syngas stream 10 (as exampled in FIG. 2), for example, in a range offrom about 5 weight percent to 25 weight percent of the raw syngasstream 10, and preferably in a range of from 10 to 20 weight percent ofthe raw stream 10. The waste gas stream 26 can be exported for fuel gasvalue.

Referring to FIG. 5, another embodiment of an ammonia manufacturingprocess can include catalytic reforming of a feed including hydrocarbon100 and steam 102 in a conventional primary reformer 122 followed byadditional reforming with excess air 106 in conventional secondarycatalytic reformer 124. Shift conversion and carbon dioxide removal 110,methanation and drying 112, syngas purification 114, compression 116,ammonia synthesis 118 and purge stream 120 recycle are as described inreference to FIG. 4. Waste gas stream 26 can be burned as a fuel inprimary reformer 122 and/or exported for fuel gas as in FIG. 4.

The purification process of FIG. 2 can be used in a new plant forimproved energy consumption and capital cost savings, or can be used toretrofit an existing purification process like that of FIG. 1 to reduceoperating costs and/or to increase capacity. The process of FIG. 2 canbe used to retrofit an existing plant that does not use purificationand/or excess air. Retrofitting for reforming with excess air canincrease the capacity of the existing plant and enhance the life of thetubes and/or other internals in the existing reformer(s) by shiftingsome of the reforming duty to the secondary reformer and lowering theoperating temperature of the primary reformer. Installing nitrogenremoval also allows for more flexible reforming operation (such as,higher methane slip), and less purge or recycle from the ammoniasynthesis loop due to the reduction of inerts with the nitrogen removal.Nitrogen purification/excess air retrofits using the low-ΔP purificationprocess of the present invention can improve the retrofit by reducing oreliminating the extent of modifications to the makeup syngas compressor,which can make the retrofit economically feasible for a larger number ofexisting ammonia plants.

EXAMPLE

The purification method of the present invention embodiment of FIG. 2 iscompared to that of the prior art in FIG. 1. Both FIGS. 1 and, 2 processa raw syngas feed stream 10 to produce a purified syngas stream 24 and awaste gas stream 26, and the inlet and outlet stream compositions arethe same in both cases as shown in Table 1 below. TABLE 1 PurificationSyngas Specifications Stream Composition, mole percent Raw PurifiedWaste Gas Syngas Syngas Gas Component (10) (24) (26) Hydrogen 65.8 74.76.6 Nitrogen 31.4 24.9 74.2 Methane 2.2 0.006 16.7 Argon 0.6 0.4 2.5Total 100.0 100.0 100.0

Operation of the low-ΔP process of FIG. 2 was simulated for a 2200metric tons per day ammonia plant to compare the operating temperatures,pressures and flow rates to those of the FIG. 1 prior art process as abase case. The results are shown in Table 2 below. TABLE 2 PurificationOperating Conditions Basis: 2200 MTPD Ammonia Base Case Example ProcessStream, Location (FIG. 1) (FIG. 2) RAW SYNGAS (10), INLET TOCROSS-EXCHANGER (20) Temperature, ° C. 4.0 4.0 Pressure, kPa 3,479.03,479.0 Mass flow, kg/hr 142,124 142,124 RAW SYNGAS (10), INLET TOCOLUMN (20) Temperature, ° C. −172.6 −172.0 Pressure, kPa 3,240.03,454.0 Mass flow, kg/hr 142,124 142,124 SYNGAS (24), OUTLET FROM COLUMN(20) Temperature, ° C. −178.6 −178.2 Pressure, kPa 3,215.0 3,429.0 Massflow, kg/hr 99,607 99,529 SYNGAS (24), OUTLET FROM CROSS-EXCHANGER (16,20) Temperature, ° C. 1.3 2.1 Pressure, kPa 3,165.0 3,404.0 Mass flow,kg/hr 99,607 99,529 BOTTOMS LIQUID (28), OUTLET FROM COLUMN (20)Temperature, ° C. −172.8 −172.2 Pressure, kPa 3,240.0 3,454.0 Mass flow,kg/hr 42,517 42,596 WASTE FLUID (26), INLET TO EXCHANGER (32)Temperature, ° C. −186.0 −187.6 Pressure, kPa 319.0 302.1 Mass flow,kg/hr 42,517 42,596 WASTE FLUID (26), OUTLET FROM CROSS-EXCHANGER (16,36) Temperature, ° C. 1.3 2.1 Pressure, kPa 256.4 253.3 Mass flow, kg/hr42,517 42,596

The data in Table 2 show that the flow rates and temperatures aresimilar, but the pressure drop for the syngas between the purificationprocess inlet and outlet is considerably lower in the FIG. 2 examplecompared to the FIG. 1 base case. This will generally require lessmakeup gas compression to the ammonia synthesis loop pressure. The powerrequirements for makeup syngas compression, fluid expansion poweroutput, and net compression and expansion were also determined for theFIG. 1 base case and the FIG. 2 example. The results are shown in Table3 below. TABLE 3 Power Balance Basis: 2200 MTPD Ammonia Base CaseExample Compression/Expansion (FIG. 1) (FIG. 2) MAKEUP SYNGASCOMPRESSION, KW 8,310.66 7,453.49 RAW SYNGAS EXPANSION, KW −203.39 —WASTE FLUID EXPANSION, KW — −120.40 NET COMPRESSION/EXPANSION 8,107.277,333.09 POWER, KW

As seen in the data presented above, the purification process of FIG. 2is characterized by a lower syngas pressure drop than the prior artprocess of FIG. 1. While less power is recovered from expansion of thewaste fluid in the example of FIG. 2 than in the syngas feed expansionin the base case of FIG. 1, the reduction in makeup compression power ismore significant. Thus, not only is the syngas pressure drop reduced,but the overall power requirements are also less, potentially resultingin both capital and operating cost savings in a new ammonia plant. In aretrofit of an existing non-purifier based ammonia plant, the reducedpressure drop of the FIG. 2 example can result in increased capacityand/or less significant or no modification of the makeup syngascompressor.

The embodiments are described above with reference to non-limitingexamples provided for illustrative purposes only. Various modificationsand changes will become apparent to the skilled artisan in view thereof.All such changes and modifications are intended within the scope andspirit of the appended claims and shall be embraced thereby.

1) An apparatus for purifying a raw syngas stream containing excessnitrogen comprising: means for introducing the raw syngas stream to afeed zone in a distillation column; means for expanding a liquid bottomsstream from the distillation column through a liquid expander with awork output to form a cooled waste fluid stream; means for rectifyingvapor from the feed zone in the distillation column to form an overheadvapor stream of reduced nitrogen and inerts content; means for coolingthe overhead vapor stream in indirect heat exchange with the cooledwaste fluid stream to form a partially condensed overhead stream and arelatively warm waste fluid stream; means for separating the partiallycondensed overhead stream into a condensate stream and a purified syngasvapor stream of reduced nitrogen and inerts content; and means forrefluxing the distillation column with the condensate stream. 2) Theapparatus of claim 1, further comprising means for cooling and expandingthe raw syngas stream across a Joule-Thompson valve in advance of theintroduction to the feed zone. 3) The apparatus of claim 2, wherein themeans for expanding the liquid bottoms stream comprises a hydraulicturbine. 4) The apparatus of claim 1, wherein the relatively warm wastefluid stream from the overhead vapor cooling consists of a vapor phase.5) The apparatus of claim 1, wherein the relatively warm waste fluidstream from the liquid expander comprises mixed vapor and liquid. 6) Theapparatus of claim 1, wherein a liquid level in the distillation columnis controlled by adjusting flow the expansion to the liquid bottomsstream. 7) The apparatus of claim 1, further comprising means forproducing the raw synthesis gas by reforming a hydrocarbon, wherein themeans for producing the raw synthesis gas comprises autothermal orsecondary reforming with excess air. 8) The apparatus of claim 1,further comprising means for supplying the purified syngas vapor streamto an ammonia synthesis loop to form ammonia. 9) An ammonia processplant comprising: means for reforming a hydrocarbon to form syngas,wherein the reforming means include an autothermal or secondary reformerand means for supplying excess air to the autothermal or secondaryreformer to form a raw syngas stream containing excess nitrogen forammonia synthesis; cross-exchanger means for cooling the raw syngasstream; means for expanding the cooled raw syngas stream from thecross-exchanger; means for introducing the expanded raw syngas stream toa feed zone in a distillation column; means for expanding a liquidbottoms stream from the distillation column through a liquid expanderwith a work output to form a cooled waste fluid stream; means forrectifying vapor from the feed zone in the distillation column to forman overhead vapor stream of reduced nitrogen and inerts content; meansfor cooling the overhead vapor stream in indirect heat exchange with thecooled waste fluid stream to form a partially condensed overhead streamand a partially warmed waste fluid stream; means for separating thepartially condensed overhead stream into a condensate stream and apurified syngas vapor stream of reduced nitrogen and inerts content;means for refluxing the distillation column with the condensate stream;means for heating the purified syngas vapor stream in thecross-exchanger; means for heating the partially warmed waste fluidstream in the cross exchanger; means for supplying the purified syngasvapor stream from the cross-exchanger to an ammonia synthesis loop. 10)The ammonia process plant of claim 9, further comprising means forcooling and expanding the raw syngas stream across a Joule-Thompsonvalve in advance of the introduction to the feed zone. 11) The ammoniaprocess plant of claim 10, wherein the means for cooling of the rawsyngas stream includes cross-exchange against the partially warmed wastefluid stream and against the purified syngas vapor stream. 12) Theammonia process plant of claim 9, wherein the means for expanding theliquid bottoms stream comprises a hydraulic turbine. 13) The ammoniaprocess plant of claim 9, wherein the partially warmed waste fluidstream from cooling of the overhead vapor stream consists of a vaporphase. 14) The ammonia process plant of claim 9, wherein waste fluidfrom the liquid expander comprises mixed vapor and liquid. 15) Theammonia process plant of claim 9, wherein a liquid level in thedistillation column is controlled by adjusting flow the expansion to theliquid bottoms stream. 16) The ammonia process plant of claim 9, furthercomprising means for producing the raw synthesis gas by reforming ahydrocarbon, wherein the means for producing the raw synthesis gascomprises autothermal or secondary reforming with excess air. 17) Theammonia process plant of claim 9, further comprising means for supplyingthe purified syngas vapor stream to an ammonia synthesis loop to formammonia.